RESEARCH ARTICLE Wood Chemical Composition in Species of Cactaceae: The Relationship between Lignification and Stem Morphology Jorge Reyes-Rivera1☯, Gonzalo Canché-Escamilla2‡, Marcos Soto-Hernández3‡, Teresa Terrazas1☯* 1 Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México, Mexico City, Mexico, 2 Unidad de Materiales, Centro de Investigación Científica de Yucatán, Mérida, Yucatán, Mexico, 3 Programa de Botánica, Colegio de Postgraduados en Ciencias Agrícolas, Montecillo, Estado de México, a11111 Mexico ☯ These authors contributed equally to this work. ‡ These authors also contributed equally to this work. * [email protected] OPEN ACCESS Abstract Citation: Reyes-Rivera J, Canché-Escamilla G, In Cactaceae, wood anatomy is related to stem morphology in terms of the conferred sup- Soto-Hernández M, Terrazas T (2015) Wood port. In species of cacti with dimorphic wood, a unique process occurs in which the cambi- Chemical Composition in Species of Cactaceae: The um stops producing wide-band tracheids (WBTs) and produces fibers; this is associated Relationship between Lignification and Stem Morphology. PLoS ONE 10(4): e0123919. with the aging of individuals and increases in size. Stem support and lignification have only doi:10.1371/journal.pone.0123919 been studied in fibrous tree-like species, and studies in species with WBTs or dimorphic Academic Editor: Haibing Yang, Purdue University, wood are lacking. In this study, we approach this process with a chemical focus, emphasiz- UNITED STATES ing the role of wood lignification. We hypothesized that the degree of wood lignification in Received: August 29, 2014 Cactaceae increases with height of the species and that its chemical composition varies with wood anatomy. To test this, we studied the chemical composition (cellulose, hemicellu- Accepted: March 9, 2015 lose, and lignin content) in 13 species (2 WBTs wood, 3 dimorphic, and 8 fibrous) with con- Published: April 16, 2015 trasting growth forms. We also analyzed lignification in dimorphic and fibrous species to Copyright: © 2015 Reyes-Rivera et al. This is an determine the chemical features of WBTs and fibers and their relationship with stem sup- open access article distributed under the terms of the port. The lignin contents were characterized by Fourier transform infrared spectroscopy and Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any high performance liquid chromatography. We found that 11 species have a higher percent- medium, provided the original author and source are age (>35%) of lignin in their wood than other angiosperms or gymnosperms. The lignin credited. chemical composition in fibrous species is similar to that of other dicots, but it is markedly Data Availability Statement: All relevant data are heterogeneous in non-fibrous species where WBTs are abundant. The lignification in WBTs within the paper and its Supporting Information files. is associated with the resistance to high water pressure within cells rather than the contribu- Funding: The authors received funding from Project tion to mechanical support. Dimorphic wood species are usually richer in syringyl lignin, and IN209012 Programa de Apoyo a Proyectos de tree-like species with lignified rays have more guaiacyl lignin. The results suggest that wood Investigación e Innovación Tecnológica, DGAPA, Universidad Nacional Autónoma de México TTand anatomy and lignin distribution play an important role in the chemical composition of wood, CONACYT (220343) to JRR. The funders had no role and further research is needed at the cellular level. in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. PLOS ONE | DOI:10.1371/journal.pone.0123919 April 16, 2015 1/17 Wood Chemical Composition in Species of Cactaceae Introduction Within angiosperms, the Cactaceae family is one of the most variable in terms of morphology and stem size, and this diversity is correlated with the anatomical features of the wood (Fig 1). Members of Cactaceae have both fibrous and non-fibrous wood [1] and are also dimorphic in the Cactoideae subfamily [2]. Cactaceae have tree-like species (hereafter called taller species) that are primarily fibrous, as observed in Pereskia lychnidiflora A.P. de Candolle, Stenocereus Fig 1. Phylogeny of Cactaceae. Asterisks indicate the analyzed species, which represent from the early derived taxa to the more derived ones, the different stem morphologies (tree, shrub, columnar, cylindrical, globose depressed) and the types of wood (fibrous, non fibrous, dimorphic). Topology based on and modified from references [58], [59]. doi:10.1371/journal.pone.0123919.g001 PLOS ONE | DOI:10.1371/journal.pone.0123919 April 16, 2015 2/17 Wood Chemical Composition in Species of Cactaceae dumortieri (Scheidw.) Backeb., and Opuntia streptacantha Lemaire. The cylindrical, globose or globose-depressed species (hereafter called smaller species) have wood with abundant wide- band tracheids (WBTs) or with few fibers and parenchyma, such as Coryphantha clavata (Scheidw.) Backeb., as well as others with dimorphic wood (the change from WBTs to fibers as the predominant cell type) in medium-sized species such as Echinocactus platyacanthus Link & Otto and Ferocactus histrix (DC.) G.E. Linds. [3], [4]. The wood anatomy in Cactaceae is well documented, and several authors have noted a posi- tive correlation between the characteristics of the wood (e.g., length and diameter of the fibers or vessel elements) and the plant morphology in terms of the support required to support the stem [4]-[8]. In some species of Cacteae, the traits of secondary xylem and the cell types present in the wood vary with the size of the species [9]. WBTs play a key role in the biology of cacti be- cause these specialized cells occur in almost all species and may or may not persist through the life of the plant: in species with dimorphic wood, the vascular cambium stops producing WBTs and produces fibers. This is a unique process to cacti and is related to the aging of individuals and the improvement of stem support [10], [11]. It is important to note that these cells are structurally different from tracheids or fiber-tracheids of other linages because WBTs have thin primary walls and annular or helical secondary walls that project deeply into the tracheid lumen. It remains unknown if the chemical features of the wood in species with an abundance of WBTs or with dimorphic wood are similar to those of the fibrous species and if these are re- lated to the stem morphology. In recent years, studies have focused on the wood chemical composition of economically important species, and it is well documented that properties such as strength, rigidity and de- fense are principally enhanced by the lignin of the secondary cell walls and the anatomical fea- tures of the wood [12], [13]. In addition, previous studies have shown that the percentage and composition of lignin vary across different plant groups (i.e., 19–24% in angiosperms; 27–33% in gymnosperms) [14]-[17]. In dicots, lignin is usually composed of moieties of guaiacyl (G) and syringyl (S); in monocots, lignin also occurs as p-hydroxyphenyl (H) units. Lignin in gym- nosperms and pteridophytes is richer in guaiacyl [13], [14], [18]. Similarly, the topochemical distribution of lignin in the wood varies with cell type. Previous research has shown that the fi- bers are richer in syringyl lignin (S), whereas guaiacyl lignin (G) is more concentrated in the secondary walls of vessels [19]-[21]. However, the variation in the lignification of the wood is influenced by anatomical characteristics. For example, the woody angiosperms have fibers, ves- sels and parenchyma and contain less lignin than gymnosperms whose wood contains only tra- cheids and scarce parenchyma [22], [23]. Because tracheids and fibers constitute the majority of wood cells in gymnosperms and angiosperms, respectively, they provide a significant contri- bution to the physical and chemical properties of the wood and stems [23]. However, the role of lignification in the support of the stem within Cactaceae is not currently well understood be- cause most species develop WBTs that differ structurally from cells present in other lineages; to date, these WBTs have not been chemically analyzed. Here, we study the relationship between stem support and wood anatomy in Cactaceae with a chemical focus, emphasizing the role of wood lignification. We hypothesized that the degree of wood lignification increases with the height of the spe- cies and that its chemical composition varies according to the anatomical characteristics of the wood in terms of the abundance of WBTs or fibers. To test this, we quantified the chemical components of wood in thirteen species of Cactaceae with different sizes and different growth forms. We characterized the lignin by Fourier transform infrared spectroscopy (FTIR) and high performance liquid chromatography (HPLC) and determined whether the percentage of lignin is related to the abundance of a specific cell type or stem morphology. PLOS ONE | DOI:10.1371/journal.pone.0123919 April 16, 2015 3/17 Wood Chemical Composition in Species of Cactaceae Materials and Methods We studied the chemical composition of wood in adult plants with contrasting growth forms belonging to thirteen species from three subfamilies of Cactaceae (Fig 1), representing from the early derived